Cardiovascular exercise, also known as cardiorespiratory exercise or aerobic exercise focuses on the human body's use of oxygen in metabolic processes and strengthening slow-twitch muscles. To stimulate these physiological processes, cardiovascular exercises are performed at moderate levels of intensity for extended periods of time, relative to anaerobic exercises which focus on building fast-twitch muscle. Cardiovascular exercise has been linked to many benefits including the prevention of heart disease and diabetes, rebuilding of lung tissue after quitting smoking, improved circulation, reduction of cholesterol and fat, more efficient use of oxygen, increased endurance, improved mental health, and a greater life span.
Perhaps the oldest and most popular form of cardiovascular exercise is running. Scientists estimate that the human body first developed the ability to run roughly four and a half million years ago in order to hunt and escape from animals. Evidence of running as a sport dates back, at least, to the Tailteann Games of Ireland in 1829 BC. Now, running is a ubiquitous sport with a place in the Olympics, high school and college-level athletics, various marathons around the world, and casual running. As compared to other cardiovascular exercises, such as swimming, in-line skating, or bicycling, it can be done almost anywhere and requires no special equipment. However, running outdoors has certain drawbacks that running on specialized equipment such as a treadmill does not.
Treadmills, which simulate a moving terrain, in many instances give the runner the ability to set the speed as well as the incline or decline of the terrain. Further, a treadmill can be located in a relatively small space, making indoor running possible. This is advantageous because indoor conditions can be controlled whereas outdoor conditions, such as rain, snow, ice, and extreme temperatures can make running uncomfortable or unsafe. A further advantage provided by mechanisms integrated into certain expensive high-end treadmills is providing a simulated terrain that gives under the weight of a runner more than many outdoor surfaces do. Given that running as a cardiovascular exercise involves repeated impact over an extended period of time, reducing the impact on the body from each stride decreases the likelihood of impact-related injuries such as patellar tendonitis.